Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. By a wide margin the most commonly employed photographic emulsions are silver bromide and bromoiodide emulsions. Although known and used for many years for selected photographic applications, the more rapid develop-ability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride, based on total silver.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid develop-ability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
The various photographic advantages were initially associated with achieving high aspect ratio tabular grain emulsions. As herein employed and as normally employed in the art, the term "high aspect ratio tabular grain emulsion" is defined as a photographic emulsion in which tabular grains having a thickness of less than 0.3 .mu.m and an average aspect ratio of greater than 8 account for at least 50 percent of the total grain projected area of the emulsion. Aspect ratio is the ratio of tabular grain effective circular diameter (ECD) divided by tabular grain thickness (t).
In reviewing the various components of the high aspect ratio tabular grain emulsion definition it is apparent that the average aspect ratio of an emulsion can be raised by increasing the ECD of the tabular grains while maintaining tabular grain thicknesses up to the 0.3 .mu.m limit. Once the practical value of tabular grain emulsions was appreciated, the average aspect ratios of the emulsions were soon raised by increasing tabular grain ECD's to their useful limits, based on acceptable levels of granularity. In fact, the earliest patents required the tabular grains to have an ECD of at least 0.6 .mu.m. Thus, the most dramatic initial impact of high aspect ratio tabular grain emulsions was in high speed photographic applications--e.g., at or above 1000 ASA speed ratings.
It was subsequently recognized that the advantages of tabular grain emulsions were also significant at even moderate average aspect ratios. Moderate aspect ratio tabular grain emulsions are herein defined and normally recognized in the art to embrace photographic emulsions in which tabular grains having a thickness of less than 0.3 .mu.m and an average aspect ratio of at least 5 account for at least 50 percent of the total grain projected area of the emulsion. At present tabular grain emulsions are recognized to include average aspect ratios as low as 2.
A difficult to achieve improvement was realized by increasing the percentage of the total grain projected area accounted for by the tabular grain population. This required developing a better understanding and control of the conditions under which tabular grains were formed, particularly the conditions of nucleation and twin plane formation. Gradually the capability of precipitating emulsions with the desired tabular grain population accounting for much more than 90 percent of the total grain projected area has been realized.
In considering further improvement of tabular grain emulsions intended for high speed photographic applications and in considering extending their advantages to moderate and slower speed photographic applications, the realization has occurred that maximizing the photographic advantages of tabular grain emulsions hinges on being able to satisfy tabular grain percent projected area and average aspect ratio requirements with the thinnest possible tabular grain population.
This realization has led to efforts to produce tabular grain emulsions containing ultrathin tabular grains. By "ultrathin" it is meant that the tabular grains have a thickness of less than 360 {111} crystal lattice planes. The spacing between adjacent {111} AgCl crystal lattice planes is 1.6 .ANG.. Daubendiek et al U.S. Pat. Nos. 4,672,027 and 4,693,964 report the preparation of ultrathin high aspect ratio tabular grain silver bromide and silver bromoiodide emulsions.
Maskasky U.S. Pat. No. 5,217,858 (hereinafter referred to as Maskasky I) was the first to succeed in preparing a high chloride ultrathin tabular grain emulsion. This was accomplished by employing a 4,6-di(hydroamino)-5-aminopyrimidine grain growth modifier, preferably one satisfying the following formula: ##STR1## where N.sup.4, N.sup.5 and N.sup.6 are amino moieties independently containing hydrogen or hydrocarbon substituents of from 1 to 7 carbon atoms, with the proviso that the N.sup.5 amino moiety can share with each or either of N.sup.4 and N.sup.6 a common hydrocarbon substituent completing a five or six member heterocyclic ring. Examples of high chloride ultrathin tabular grain emulsions were provided using 4,5,6-triaminopyrimidine and adenine as representative grain growth modifiers satisfying formula I.
Maskasky U.S. Pat. No. 5,178,997 (hereinafter designated Maskasky II) discloses a process for preparing a high chloride tabular grain emulsion in which silver ion is introduced into a gelatino-peptizer dispersing medium containing a stoichiometric excess of chloride ions of less than 0.5 molar and a 7-azaindole type grain growth modifier of the formula: ##STR2## where Z.sup.2 is --C(R.sup.2).dbd. or --N.dbd.;
Z.sup.3 is --C(R.sup.3).dbd. or --N.dbd.; PA1 Z.sup.4 is --C(R.sup.4).dbd. or --N.dbd.; PA1 Z.sup.5 is --C(R.sup.5).dbd. or --N.dbd.; PA1 Z.sup.6 is --C(R.sup.6).dbd. or --N.dbd.; PA1 with the proviso that no more than one of Z.sup.4, Z.sup.5 and Z.sup.6 is --N.dbd.; PA1 R.sup.2 is H, NH.sub.2 or CH.sub.3 ; PA1 R.sup.3, R.sup.4 and R.sup.5 are independently selected, R.sup.3 and R.sup.5 being hydrogen, hydroxy, halogen, amino or hydrocarbon and R.sup.4 being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbon atoms; and PA1 R.sup.6 is H or NH.sub.2. PA1 Z.sup.3 is --C(R.sup.3).dbd. or --N.dbd.; PA1 Z.sup.4 is --C(R.sup.4).dbd. or --N.dbd.; PA1 Z.sup.5 is --C(R.sup.5).dbd. or --N.dbd.; PA1 Z.sup.6 is --C(R.sup.6).dbd. or --N.dbd.; PA1 with the proviso that no more than one of Z.sup.4, Z.sup.5 and Z.sup.6 is --N.dbd.; PA1 R.sup.2 is H, NH.sub.2 or CH.sub.3 ; PA1 R.sup.3, R.sup.4 and R.sup.5 are independently selected, R.sup.3 and R.sup.5 being hydrogen, hydroxy, halogen, amino or hydrocarbon and R.sup.4 being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbon atoms; and PA1 R.sup.6 is H or NH.sub.2. PA1 360 lattice planes&lt;0.06 .mu.m PA1 300 lattice planes&lt;0.05 .mu.m PA1 180 lattice planes&lt;0.03 .mu.m PA1 120 lattice planes&lt;0.02 .mu.m PA1 Z.sup.3 is --C(R.sup.3).dbd. or --N.dbd.; PA1 Z.sup.4 is --C(R.sup.4).dbd. or --N.dbd.; PA1 Z.sup.5 is --C(R.sup.5).dbd. or --N.dbd.; PA1 Z.sup.6 is --C(R.sup.6).dbd. or --N.dbd.; PA1 with the proviso that no more than one of Z.sup.4, Z.sup.5 and Z.sup.6 is --N.dbd.; PA1 R.sup.2 is H, NH.sub.2 or CH.sub.3 ; PA1 R.sup.3, R.sup.4 and R.sup.5 are independently selected, R.sup.3 and R.sup.5 being hydrogen, hydroxy, halogen, amino or hydrocarbon and R.sup.4 being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbon atoms; and PA1 R.sup.6 is H or NH.sub.2. PA1 R.sup.8 is H, NH.sub.2 or CH.sub.3 ; and PA1 R.sup.1 is hydrogen or a hydrocarbon containing from 1 to 7 carbon atoms.
Maskasky II contains no disclosure of high chloride ultrathin tabular grain emulsions.
Jones et al U.S. Pat. No. 5,176,991 discloses a process of chemically sensitizing high chloride tabular grain emulsions, including those prepared as taught by Maskasky I and II. Protonation of the formula I or II compound adsorbed to the grain surfaces is initiated, chemical sensitization is performed while protonation is occurring, and protonation is then terminated, so that at least a portion of the adsorbed formula I or II compound is retained on the surfaces of the sensitized grains.